Double crucible system for the production of light conducting fibers

ABSTRACT

A double crucible system is disclosed for the production of light conducting fibers. The system has an interior crucible which can be axially displaced in relation to the exterior crucible which is concentric thereto. The spacing of the crucible jets in the exterior and interior crucible in relation to one another is thereby altered so that the proportion of the cross-sections of the fiber core and the fiber casing is continuously altered.

This is a division of application Ser. No. 852,236, filed Nov. 17, 1977,now U.S. Pat. No. 4,118,212, granted Oct. 3, 1978.

BACKGROUND OF THE INVENTION

The invention relates to a double crucible system for the production oflight conducting fibers.

It is already known to draw light conducting fibers from a doublecrucible. For example, the publicaton by H.G. Unger, "OptischeNachrichtentechnik", 1976 Berlin, describes a device of this type underthe key word "double nozzle method" on pages 39 and 40.

Previously, light conducting fibers produced in accordance with thedouble crucible method possessed a predetermined ratio between thecross-sections of the fiber cores and the fiber casings. This ratio ofthe cross-sections was determined by the structure of the doublecrucible. In order to produce fibers having a different ratio of thecasing and core cross-sections, it was previously necessary to employ adifferent double crucible.

As the double crucibles are generally manufactured from highly pureplatinum or platinum-rhodium alloys in order to avoid pollution of theglass melts by the crucible material, double crucibles of this type areextremely expensive and this has an unfavorable influence upon fibercost.

SUMMARY OF THE INVENTION

An object of the invention is to provide a double crucible device withwhich it is possible to produce light conducting fibers in which theratio between core and casing cross-sections can be set virtuallyarbitrarily.

This object is realized by a double crucible system which, in accordancewith the invention, provides for an adjustment of the spacing betweenconcentric nozzles on inner and outer crucibles.

Thus, in accordance with the invention, the two crucibles of the doublecrucible are axially displaceable relative to one another. As a result,the distance between the crucible nozzles of the two crucibles relativeto one another changes, and accordingly the ratio of the cross-sectionsof the fiber core to the fiber casing changes also. The greater thedistance between the crucible nozzles, the smaller is the corecross-section relative to the casing cross-section.

Advantageously, monomode and mulitmode fibers can be produced with thedouble crucible system in accordance with the invention. In monomodefibers, the core has a particularly small cross-section relative to thecasing, and furthermore the core possesses only a slightly higher indexof refraction than the fiber casing. In multimode fibers, on the otherhand, the core has a relatively large cross-section and the differencebetween the indices of refraction between the core and casing isrelatively great.

In an advantageous embodiment of the double crucible system, in additionto a change in the distance between the two crucibles, it is alsopossible for the inner crucible to rotate about its axis. In this way anagitation effect can advantageously be achieved. As a result it ispossible to eliminate gas bubbles which can settle on the crucible wallsduring the filling of the crucibles.

This agitation effect also has a favorable influence upon thehomogenization of the temperature in the glass melt. Temperaturefluctuations in the melt can lead to undesirable changes in index ofrefraction. As these fluctuations can be avoided by the cruciblerotation, light conducting fibers exhibiting very low scattering lossescan be produced with a double crucible system of this kind.

Advantageously the double crucible system can be designed in such amanner that the crucibles can be continuously loaded, even during therotation of the crucibles, so that it is possible to produce lightconducting fibers having arbitrary lengths.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE illustrates an exemplary embodiment in partial cross-sectionof a double crucible system in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with the invention, the double crucible system possessesan outer crucible 1 and an inner crucible 2. The outer crucible containsa glass melt 3, and the inner crucible a glass melt 4. In order to avoidpollution of the surfaces of the glass melts, and in order to be able tocheck the atmosphere over the melts, the double crucible is arranged ina capsule 5 consisting of quartz glass. The crucible can be heated withan induction coil 6. The outer crucible 1 is suspended on a water-codedflange 7. The inner crucible 2 is suspended on a tubular shaft 8 which,in accordance with an advantageous embodiment, is rotatable as indictedby corresponding arrows. By means of a shaft suspension 9 which isadjustable in height, the shaft and the inner crucible can be adjustedwith respect to their height relative to the outer crucible. In this wayit is possible to adjust the distance of the nozzle 10 of the outercrucible relative to the nozzle 20 of the inner crucible. The cruciblescan be loaded through glass rods 30, 40. These glass rods which, forexample, have a diameter of 4 mm to 10 mm, are slowly inserted into thecrucibles. The glass rod 40 is inserted into the inner crucible throughthe tubular shaft 8.

The crucible nozzles can be closed off by means of a seal (notillustrated). As soon as the glass melts have been brought to thedrawing temperature, this seal is removed so that a light conductingfiber 11 can be drawn. A drawing drum 15 is provided for this purpose.

A thickness measuring device 12 can be provided for measuring thethickness of the light conducting fibers.

If it is desired to provide the light conducting fibers with anadditional protective casing, a coating bath 13 can be provided.Polyvinyliden chloride (trade name: Kynar) is a suitable material, forexample, for a protective casing. A drying furnace 14 can be used toachieve a more rapid hardening of the protective casing.

The crucibles 1 and 2 must consist of a material which does not pollutethe glass melts in the crucible. Suitable materials for the cruciblesare, for example, platinum and iridium, which metals can also be alloyedwith rhodium.

First investigations have indicated that highly pure aluminum oxide isalso suitable as a crucible material. As yet no corrosion phenomena havebeen able to be established in this material and the pollution in theglass melt caused by this crucible material is negligible. Glass typesof the system Na₂ O--K₂ O--PbO--SiO₂ have been employed for testing thedouble crucible system in accordance with the invention.

These glass types possess the desired property of exhibiting no tendencyto crystallization within a wide temperature range, as known from thepublication by W. Vogel, "Struktur und Kristallisation der Glaser",Lepzig 1965. Furthermore, these glass types can be drawn at relativelylow temperatures, this being advantageous as it is thus possible tomaintain pollution due to the crucible material at a particularly lowlevel. Furthermore, with these types of glass, it is possible to changethe index of refraction within a wide range by altering the PbOconcentration. Thus it is easily possible, with these types of glass, toachieve a wide difference in index of refraction between the casing andthe core of the later formed light conducting fiber. Furthermore theseglass types have a good chemical and thermal stability and furthermoregood mechanical properties.

The glass rods 30 and 40 provided for the loading of the crucibles canbe manufactured in the following manner. The highly pure raw materialsof the glass system are fused in the desired mixture ratio in platinumcrucibles or in highly pure ceramic crucibles. This fusion process iscarried out in an argon-oxygen atmosphere, keeping the melt times asshort as possible in order to ensure that the melt is polluted by thecrucible material to the least extent possible. The glass melt ishomogenized by agitation with a platinum rod or ceramic rod. Now glassrods can be directly drawn out of a crucible opening. During testing,glass rods having a thickness of between 4 and 10 mm and a length of upto 1 m were used. The fluctuation in thickness of the rods was in theregion of ±3%.

Absorption measurements carried out on the drawn light conducting fibershave indicated that the absorption losses of the light conducting fibersare dependent both upon the purity of the raw materials and upon thecrucible material and the atmosphere employed during the meltingprocess.

It has been shown that glasses melted in platinum crucibles may assume ayellow color. However, the glass melts can be rendered transparent againby blowing oxygen through the melt. The yellow color is probably due toplatinum dissolved in colloidal form, which is oxidized by the oxygen,thus rendering the melt transparent again.

Obviously the platinum is extremely homogeneously distributed in themelt, which means that the platinum does not produce any scatteringcenters within the drawn light conducting fiber.

When crucibles consisting of iridium and a protective atmosphereconsisting of argon were used to which were added a few percent ofoxygen, a colorless glass melt was always achieved. However, inclusionsof iridium or iridium oxides in the glass melt result in a relativelyhigh number of scattering centers which led to relatively high losses inthe later formed light conducting fiber, although these losses can bedisregarded in the case of transmission paths of not too great a length.

The glass rods which have been produced in this way are now introducedinto the double crucible system and are fused. A protection frompollution is provided by a protective atmosphere, e.g. argon or nitrogenin the capsule 5. As soon as the glass melts have been brought todrawing temperature, a light conducting fiber can be drawn off.

The double crucible system in accordance with the invention canadvantageously also be used to produce gradient index profile lightconducting fibers. For this purpose melts in which a sufficiently strongdiffusion occurs at a common boundary surface are used for the fibercore and the fiber casing. In particular, between the nozzles of thedouble crucible, in a high temperature range, the melts partiallydiffuse into one another at their common boundary surface. By adjustingthe spacing between the nozzles--this fundamentally governs the periodof time in which diffusion takes place--the index of refraction profilecan be caused to become parabolic, for example.

The double crucible system in accordance with the invention and theaforementioned types of glass are particularly suitable for theproduction of light conducting fibers whose absorption loss lies at afew dB/km, and the numerical aperture of which is 0.2 or higher. Lightconducting fibers having lower absorption losses are in fact importantfor optical long distance traffic communications systems, although inmany applications the degree of the absorption losses is less importantthan a high numerical aperture. Such applications consist, for example,of data processing, close range traffic communications systems, and airtraffic applications. For a range of, for example 100 m, a lightconducting fiber possessing an absorption of 50 dB/km and a numericalaperture of 0.47, in combination with a luminescence diode, permits thetransmission of a considerably higher light power than when a lightconducting fiber with 5 dB/km and a numerical aperture of 0.2 are used.Furthermore, light conducting fibers having a large, numerical aperturehave the particular advantage that microbending in the light conductingfiber lead to only low additional losses.

The double crucible system in accordance with the invention is alsosuitable for the production of fibers from synthetic materials. In thiscase the crucibles can also consist of glass.

Although various minor modifications may be suggested by those versed inthe art, it should be understood that I wish to embody within the scopeof the patent warranted hereon, all such embodiments as reasonably andproperly come within the scope of my contribution to the art.

We claim as our invention:
 1. A double crucible system having twoconcentric melting crucibles for the production of a light conductingfiber, comprising: an inner crucible means for accommodating a melt fora core of the light conducting fiber, an outer crucible meanssurrounding the inner crucible means for accommodating a melt for acasing of the light conducting fiber, the inner and outer crucible meanshaving concentric nozzle means for drawing off a light conducting fiber,the inner crucible means being arranged to be adjustable in its heightrelative to the outer crucible means so that the distance between theconcentric nozzles is variable and adjusting means being providedcomprising a vertical tubular shaft connecting at a center of a topportion of the inner crucible means, guide means for slidably guidingthe tubular shaft and said tubular shaft also having a glass supplyinlet.
 2. A system as claimed in claim 1 characterized in that the guidemeans comprises a sleeve slidably receiving the tubular shaft and asuspension means is provided for moving the shaft vertically relative tothe sleeve.
 3. A system as claimed in claim 1 characterized in that theinner and outer crucible means are arranged in a capsule.
 4. A system asclaimed in claim 1 characterized in that glass rods are provided forloading the crucibles.
 5. The system of claim 1 in which a casing meanssealingly surrounds the outer crucible means, said casing means having atop portion with a sealing aperture in which said tubular shaft isslidably positioned.